In the beginning there was nothing. God said, “Let there be light!” And there was light. There was still nothing, but you could see it a whole lot better.
The micro-optical devices we considered in Chapters 6 through 9 were passive optical components: mirrors, lenses, diffractive structures, and waveguides that interacted with light but, with the possible exception of actuated micromirrors, did not require any input of electrical power. Active optical components, in contrast, are those that rely on an interaction with (typically) electrical and optical fields to generate, absorb, and modulate light. They form an important category of micro-optical structures, since most optical systems require at least a light source and a means for optical detection.
As with most of the other topics of this text, active optics is a field unto itself, or better yet, a union of numerous very extensively developed fields. We provide in this chapter an overview of the active optical components of highest relevance for micro-optics and optical microsystems, including laser diodes and LEDs, detectors, modulators, optical isolators, and liquid crystal devices. The literature is of course extensive (Fukuda, 1999; Iizuka, 2002a, 2002b; Liu, 2005; Solgaard, 2009), and we will provide further references to texts that go into greater depth in each of the following sections.
Physics of light emission
Optics without light is pretty boring. All optical systems require a source of photons to realize their functionality, and micro-optical systems have an array of suitably sized light sources at their disposal.